This invention relates to low pressure discharge lamps having "cold-cathode" type discharge electrodes and, more particularly, to a fluorescent low pressure mercury vapor discharge lamp of the "instant-start" type having a pair of cold-cathode discharge electrodes.
There are two types of cathodes predominantly used in the fluorescent lamp arts. They are both heated to their "thermionic emission temperature", the temperature at which they emit electrons, during lamp operation to provide a source of electrons to support the discharge arc. One of said cathode types is termed a "hot cathode" and is heated to its emission temperature by u heated filament and the arc discharge whereas the other type of cathode is a "cold cathode" and is heated to its emission temperature solely by the arc discharge.
The hot cathode type electrodes most commercially prevalent in the art consist of a tungsten filament coated with a suitable emitter material, for example a mixture of the oxides of barium, strontium and calcium, which readily releases electrons when heated to a temperature of about 800.degree. C.
Hot cathode electrodes are used in both "pre-heat" and "rapid-start" lamps. In preheat lamps, the electrodes are heated to their emission temperature prior to ignition of the lamp by a pre-heat current. The ample supply of electrons emitted from the hot cathodes enable the lamp to ignite at voltages of about 100-300 V. The heater current is switched off after a discharge arc is ignited between the electrodes and the high temperature necessary for free emission of electrons is maintained after ignition by ionic bombardment from the discharge. In rapid start lamps, the heater current is not turned off and continues to flow through the filament electrodes after the lamp is burning.
Cold-cathode electrodes are used in "instant-start" lamps and do not employ a heater current to generate electrons to aid in lamp starting. Instant-start lamps rely solely on a high voltage of about 400 to 1000 volts between the two electrodes to initiate a glow discharge. The glow discharge provides further heating of the electrodes causing an almost instantaneous transition to an arc discharge.
The cold cathodes predominantly used in "instant start" lamps employ a helically wound tungsten filament coated with emissive material, as with hot cathode electrodes, but are of much sturdier construction and contain significantly more emitter material. Instead of a tungsten filament, other cold cathodes known in the art employ a metallic can or holder in which a substantial quantity of emitter material is deposited, as known for example from U.S. Pat. Nos. 2,677,623 (Claude et al); 3,325,281 (Ebhardt); and 2,753,615 (Claude et al).
Fluorescent lamps having filament type hot cathodes have a life which is typically limited to about 10,000 to 20,000 hours, depending on lamp wattage, due to the fact that only a limited quantity of the emissive material can be coated on the filaments and due to evaporation and scattering of the emitter material off of the filament due to ionic bombardment from the discharge. Instant-start cold-cathode lamps, by contrast, have approximately half the life of a hot-cathode lamp of corresponding wattage because the ionic bombardment of the glow-to-arc discharge transition upon starting of these lamps causes significantly more sputtering of the emitter material from the electrode.
A problem with filament type electrodes, whether for hot or cold cathode use, is that it is difficult to provide an adequate control of the amount of emissive material provided on the coiled tungsten wire. The filament electrodes are dipped in a liquid mixture including, for example barium carbonate, strontium carbonate, and calcium carbonate along with butyl acetate, nitrocellulose, butanol and zirconium oxide. After sealing in the lamps, the dipped filaments are treated according to a treatment schedule which includes passing various levels of electric current through the filaments to heat the filaments and convert the carbonates to oxides. During this treatment, the lamps are also evacuated to remove any volatiles driven off from the emitter material. The accumulation of small variations in the length and weight of the filaments, in the liquid mixture and the amount coated on the filament, and in the treatment schedule on the assembly line contribute to undesirable variations in the actual quantity of emissive material provided on the electrode in the finished lamp. Since lamp life is very sensitive to the quantity of emissive material provided, it is very difficult to control the life distribution of the lamps so as to manufacture lamps having a very narrow life distribution.
Various fused pellet composite discharge electrodes have been proposed for both hot and cold cathode operation for fluorescent lamps. U.S. Pat. No. 3,766,423 (Menelly) shows a hot cathode electrode formed with a thermochemical sintering method by mixing tungsten with oxides of barium or with mixtures of oxides of barium, calcium and strontium. The mixture is pressed about metal leads and then heated until an exothermic reaction occurs. No yttrium oxide is present. The electrode produced has a density gradient containing 80% voids in the surface of the electrode extending down to 10% voids in the central portion of the electrode. It has been found, however, that such electrodes are very fragile and are difficult to degas because of the high porosity. U.S. Pat. No. 3,758,809 (Menelly) discloses a similarly formed composite "cold-cathode" electrode which includes an integral metal lead extending from the bottom surface thereof. The pellet has a bulk density gradient structure wherein the interior portions and exterior bottom and side portions have a higher bulk density relative to the top portion of the pellet. Furthermore, the top portion of the pellet has a rough surface as compared to the smooth surface of the exterior bottom and side surfaces.
Butter et al, U.S. Pat. No. 3,718,831 discloses yet another thermochemically sintered composite electrode having a bulk density gradient structure with an integral lead. Butter discloses that the cold cathodes of Menelly '809 were unsatisfactory because their ignition voltage was found to increase rapidly after a short burning time such that they could not be ignited on standard commercial ballasts. This was believed to be due to excessive sputtering and migration of the emitter material from the surface into the interior regions of the electrode. The electrode according to Butter has a cavity of conic section which reduces the amount of emitter material dislodged from the surface of the electrode and creates an electric field which causes migration of the emitter material to the outside surface of the electrode, where the discharge terminates on this electrode. A disadvantage, however, of the Butter electrode is its complicated shape.
Iwaya et al, U.S. Pat. No. 4,808,883 shows a discharge lamp containing a "cold-cathode" electrode formed of a semiconductor ceramic material. The electrode in this lamp contains tungsten only in an amount up to 0.8 mol % and does not contain rare earth emitter materials. Other cathode configurations using semiconductor ceramics without rare earth emitter materials are known from JP 1-63253, JP 1-63254 and JP 1-77857.
Composite electrodes are also known for high pressure discharge lamps. U.S. Pat. No. 4,303,848 (Shimizu et al) discloses a sintered electrode formed from a mixture of a high melting point metal, an emissive material of an alkaline earth metal or compound thereof, and at least one oxide of a metal selected from the group consisting of yttrium, zirconium, and aluminum. An electrode supporting rod is integrally sintered in the electrode. The electrode is formed by first mixing a base metal powder with an organic binder to form agglomerates, which are then granulated. An electron emissive powder is similarly prepared, mixed with the granulated base metal powder, and the mixture compacted at a pressure of 3 ton/cm.sup.2. Before sintering at 1400.degree.'-1600.degree. C. the compacted mixture is heated at a lower temperature for an extended period to drive off the organic binder. Because of the use of an organic binder which is later driven off, the disclosed compaction pressures and sintering temperatures, and the particle sizes of 60-180 .mu.m the Shimizu electrode would have a porosity significantly greater than 10%.